1. Field of the Invention
The present invention relates to an electrode material for a lithium secondary battery, an electrode structure employing the electrode material, and a lithium secondary battery having the electrode structure.
2. Related Background Art
Recently, an amount of a CO2 gas in air has increased leading to a green house effect, and global warming caused by the green house effect has become a concern. Countermeasures for reducing the amount of CO2 gas emission have been studied globally. For example, thermal power plants, which convert thermal energy obtained by burning fossil fuel into electric energy, exhaust a large amount of the CO2 gas. Thus, construction of additional thermal power plants has become difficult. Therefore, so-called load leveling has been proposed for effective use of electric power to meet an increasing electric power demand. That is, the electric power generated at night, which is surplus power, is stored in secondary batteries installed in general households. The stored electric power is used during daytime when consumption of the electric power increases, to thereby level load distribution. Aside from this, vehicles which run on fossil fuel exhaust NOx, SOx, hydrocarbons, or the like in addition to CO2, and such vehicles are perceived as a problem as another source of air pollutants. Electric vehicles which run by driving a motor by electricity stored in the secondary batteries have attracted attention from a view of reducing the source of air pollutants, because the electric vehicles do not exhaust air pollutants. Research and development on such electrical vehicles have been actively promoted for an early practical application. High energy density, long-life, and low cost secondary batteries have been demanded for their use in load leveling applications and electrical vehicles.
Further, early provision of small, lightweight, and high performance secondary batteries is strongly urged for their use as power supply of portable equipment such as laptop personal computers, word processors, video cameras, and cellular phones.
Various high performance secondary batteries which meet the demands described above have been proposed, and some thereof are in practical use. A typical constitution of the lithium batteries in practical use employs a carbon material for a negative electrode, a lithium-cobalt mixed oxide (LiCoO2) for a positive electrode, and an ethylene carbonate electrolyte. The lithium-cobalt mixed oxide used as the positive electrode material (positive electrode active material) has a high theoretical capacity of 280 mAh/g and a low usable capacity of 160 mAh/g. In addition, LiNiO2 and LiMn2O4 known as the positive electrode materials respectively have low usable capacities of 190 mAh/g and 150 mAh/g, similar to that of the lithium-cobalt mixed oxide. The positive electrode materials have a voltage of 3.0 V (vs. Li/Li+) or more but have a practically usable capacity of 200 mAh/g or less, which is insufficient. The expression “(vs. Li/Li+)” shows a voltage with respect to lithium.
In addition, as the positive electrode materials having a voltage of 3.0 V (vs. Li/Li+) or more, olivine materials have been proposed. For example, Japanese Patent Application Laid-Open No. 2002-117848 discloses LiFePO4 having an olivine structure and substituting a part of Fe by Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, Co, Mn, or Cr, as a positive electrode material of a secondary battery. However, those materials have a usable capacity not exceeding 200 mAh/g.
Further, Japanese Patent Application Laid-Open No. H05-325971 discloses LiBxCo1 -xO2 (0.001≦x≦0.25), which is LiCoO2 having a part of cobalt substituted with boron B, as a positive electrode material of a secondary battery. Those materials have an α-NaFeO2 structure, and thus a discharge capacity of the first charging and discharging cycle decreases with increasing boron B content at x≧0.1. At x=0.3, the discharge capacity is lower than that of the first charging and discharging cycle at x=0. Thus, the positive electrode material disclosed in Japanese Patent Application Laid-Open No. H05-325971 has a low usable capacity and an unsatisfactory energy density.
In addition, as the positive electrode materials of secondary batteries, LiMO2 (M represents Co or Ni) having an α-NaFeO2 structure is known. Those materials have a high theoretical capacity of 280 mAh/g, but low usage capacities of 160 mAh/g (when M=Co) and 190 mAh/g (when M=Ni). In addition, LiMn2O4 having a spinel structure is known, but the material has a low theoretical capacity of 150 mAh/g.
As described above, many materials have been provided conventionally as a positive electrode material (positive electrode active material) of a secondary battery (that is, lithium secondary battery). However, none of the materials is satisfactory particularly regarding the practically usable capacity. That is, a positive electrode material (positive electrode active material) having a voltage of 3.0 V (vs. Li/Li+) or more, a practically usable capacity exceeding 200 mAh/g, and a high energy density has not been found yet. Thus, early provision of such positive electrode materials is strongly urged.